Shorter crank arm - no propulsion!

I recently read something (perhaps on cycling news or velo news) about shorter crank arms actually being beneficial, despite the reduction in the turning force you are able to produce.  It is due to the top and bottom of the stroke being dead spots and the longer the crank arm the more pronounced these are.

How long has it been since your fit?  Have you eliminated any other reasons for a drop in power?  If you can't attribute this to anything else then maybe switch back to 172.5 but keep everything else from your fit as close to it is now and see if that helps.

I'm one of those princess-and-the-pea types that can feel a 2.5mm difference in crank length. That said, that doesn't mean it affects my performance detrimentally, I just feel it. I have 175s on my MTB, 172.5s on my road and tri bikes, and 170s on my fixie/track bike. I did once try some 165s on my road bike, and hated it -- no power on the hills. But the shorter cranks aren't meant for "power" in a high-torque meaning, it's all about spin, and opening the hip angle at the top of the pedal stroke in an aero position (or getting lower in front while keeping the same hip angle).

As others have suggested, though, I suspect that there are other fit parameters at play here. What else changed?

You need to change your gearing. Bigger ring up front or different cassette. 

It takes a few weeks just to get used to the feel of short cranks. I ride 165's with a 54T up front and 11-23 in back most of the time. 

The benefit is a more open hip angle to hopefully run better off the bike with a faster cadence that mimics your run or you get get into a more aggressive position.